LLDPE polyethylene finds various applications and it is used in particular for the preparation of films. This is because compared to the films obtained from conventional low density polyethylene (LDPE), LLDPE films offer better mechanical properties.
However, the production of LLDPE films presents some difficulties due mainly to the fact that the polymer does not possess a sufficiently high melt strength in the molten state, and in said molten state its viscosity is rather significant.
In order to maintain unaltered the productivity of the film extruders, they have to be modified, for example the slit of the extruder's die has to be made wider, or the temperature of said die has to be increased.
These modifications cause difficulties related with the cooling of the blown bubble at the extruder's output and a film thickness which is not homogeneous.
Moreover, the LLDPE film heat-seal does not offer good heat resistance.
In order to overcome the above inconveniences, U.S. Pat. No. 4,871,813 discloses modifying LLDPE by blending a semicrystalline copolymer of propylene with a CH.sub.2 .dbd.CHR .alpha.-olefin, where R is an alkyl radical having from 2 to 10 carbon atoms, and optionally ethylene in quantities of less than 10%. The ethylene content must always be lower than that of the .alpha.-olefin. The propylene copolymer contains from 7 to 40% by weight of .alpha.-olefin, presents a melt enthalpy lower than 75 J/g, and is added in quantities ranging from 1 to 25% by weight. Moreover, the propylene copolymers described in said patent are characterized by a particularly irregular molecular structure due to the good randomization of the comonomer and the low isotactic index. In particular, the isotactic index determined by measuring the solubility in n-heptane is lower than 65. The degree of crystallinity of the propylene copolymer is low, and less than 35%, generally ranging from 10 to 30%.
The LLDPE and propylene copolymer blend is prepared by blending the melted components in an extruder and then pelletizing the resulting product, or the mixture of the solid components, and the feeding of said mixture directly in the extruder to form the finished product.
The compositions obtained in this manner offer improved processability, and heat resistance during heat-sealing. However, little, if any, substantial change is seen in the mechanical properties.
European Patent Application 0 525 767 discloses a process for the preparation of LLDPE with improved processability by sequential polymerization in two or more gas-phase reactors equipped either with a fluid or mechanically stirred bed. According to said process, in one of the reactors ethylene and CH.sub.2 .dbd.CHR .alpha.-olefin, where R is an alkyl radical having 1 to 10 carbon atoms, are polymerized to form linear low density polyethylene (LLDPE), and in another reactor propylene and a CH.sub.2 .dbd.CHR .alpha.-olefin, where R is an alkyl radical having 2 to 10 carbon atoms, are polymerized to form a crystalline propylene copolymer having a melt enthalpy higher than 70 J/g. Compared with the mechanical blends described above, the compositions thus obtained are more homogeneous and have improved optical properties.